I. The Anatomy of a Healthy Heart
The heart (see FIG. 1B) is slightly larger than a clenched fist. It is a double (left and right side), self-adjusting muscular pump, the parts of which work in unison to propel blood to all parts of the body. The right side of the heart receives poorly oxygenated (“venous”) blood from the body from the superior vena cava and inferior vena cava and pumps it through the pulmonary artery to the lungs for oxygenation. The left side receives well-oxygenation (“arterial”) blood from the lungs through the pulmonary veins and pumps it into the aorta for distribution to the body.
The heart has four chambers, two on each side—the right and left atria, and the right and left ventricles. The atria are the blood-receiving chambers, which pump blood into the ventricles. A wall composed of membranous and muscular parts, called the interatrial septum, separates the right and left atria. The ventricles are the blood-discharging chambers. A wall composed of membranous and muscular parts, called the interventricular septum, separates the right and left ventricles.
The synchronous pumping actions of the left and right sides of the heart constitute the cardiac cycle. The cycle begins with a period of ventricular relaxation, called ventricular diastole. The cycle ends with a period of ventricular contraction, called ventricular systole.
The heart has four valves (see FIGS. 1B and 1C) that ensure that blood does not flow in the wrong direction during the cardiac cycle; that is, to ensure that the blood does not back flow from the ventricles into the corresponding atria, or back flow from the arteries into the corresponding ventricles. The valve between the left atrium and the left ventricle is the mitral valve. The valve between the right atrium and the right ventricle is the tricuspid valve. The pulmonary valve is at the opening of the pulmonary artery. The aortic valve is at the opening of the aorta.
At the beginning of ventricular diastole (i.e., ventricular filling)(see FIG. 1B), the aortic and pulmonary valves are closed to prevent back flow from the arteries into the ventricles. Shortly thereafter, the tricuspid and mitral valves open (as FIG. 1B shows), to allow flow from the atria into the corresponding ventricles. Shortly after ventricular systole (i.e., ventricular emptying) begins, the tricuspid and mitral valves close (see FIG. 1C)—to prevent back flow from the ventricles into the corresponding atria—and the aortic and pulmonary valves open—to permit discharge of blood into the arteries from the corresponding ventricles.
The opening and closing of heart valves occur primarily as a result of pressure differences. For example, the opening and closing of the mitral valve occurs as a result of the pressure differences between the left atrium and the left ventricle. During ventricular diastole, when ventricles are relaxed, the venous return of blood from the pulmonary veins into the left atrium causes the pressure in the atrium to exceed that in the ventricle. As a result, the mitral valve opens, allowing blood to enter the ventricle. As the ventricle contracts during ventricular systole, the intraventricular pressure rises above the pressure in the atrium and pushes the mitral valve shut.
FIG. 1D shows a posterior oblique cutaway view of a healthy human heart 100. Two of the four heart chambers are shown, the left atrium 170, and the left ventricle 140 (not shown are the right atrium and right ventricle). The left atrium 170 fills with blood from the pulmonary veins. The blood then passes through the mitral valve (also known as the bicuspid valve, and more generally known as an atrioventricular valve) during ventricular diastole and into the left ventricle 140. During ventricular systole, the blood is then ejected out of the left ventricle 140 through the aortic valve 150 and into the aorta 160. At this time, the mitral valve should be shut so that blood is not regurgitated back into the left atrium.
The mitral valve consists of two leaflets, an anterior leaflet 110, and a posterior leaflet 115, attached to chordae tendineae 120 (or chords), which in turn are connected to papillary muscles 130 within the left atrium 140. Typically, the mitral valve has a D-shaped anterior leaflet 110 oriented toward the aortic valve, with a crescent shaped posterior leaflet 115. The leaflets intersect with the atrium 170 at the mitral annulus 190.
In a healthy heart, these muscles and their chords support the mitral and tricuspid valves, allowing the leaflets to resist the high pressure developed during contractions (pumping) of the left and right ventricles. In a healthy heart, the chords become taut, preventing the leaflets from being forced into the left or right atria and everted. Prolapse is a term used to describe the condition wherein the coaptation edges of each leaflet initially may coapt and close, but then the leaflets rise higher and the edges separate and the valve leaks. This is normally prevented by contraction of the papillary muscles and the normal length of the chords. Contraction of the papillary muscles is simultaneous with the contraction of the ventricle and serves to keep healthy valve leaflets tightly shut at peak contraction pressures exerted by the ventricle.
II. Characteristics and Causes of Mitral Valve Dysfunction
Valve malfunction can result from the chords becoming stretched, and in some cases tearing. When a chord tears, the result is a flailed leaflet. Also, a normally structured valve may not function properly because of an enlargement of the valve annulus pulling the leaflets apart. This condition is referred to as a dilation of the annulus and generally results from heart muscle failure. In addition, the valve may be defective at birth or because of an acquired disease, usually infectious or inflammatory.
FIG. 2 shows a cutaway view of a human heart 200 with a prolapsed mitral valve. The prolapsed valve does not form a tight seal during ventricular systole, and thus allows blood to be regurgitated back into the left atrium during ventricular contraction. The anterior 220 and posterior 225 leaflets are shown rising higher than normal (i.e., prolapsing) into the left atrium. The arrows indicate the direction of regurgitant flow. Among other causes, regurgitation can result from redundant valve leaflet tissue or from stretched chords 210 that are too long to prevent the leaflets from being blown into the atrium. As a result, the leaflets do not form a tight seal, and blood is regurgitated into the atrium.
FIG. 3 shows a cutaway view of a human heart 300 with a flailing mitral valve 320. The flailing valve also does not form a tight seal during ventricular systole. Blood thus regurgitates back into the left atrium during ventricular contraction, as indicated by the arrows. Among other causes, regurgitation can also result from torn chords 310.
As a result of regurgitation, “extra” blood back flows into the left atrium. During subsequent ventricular diastole (when the heart relaxes), this “extra” blood returns to the left ventricle, creating a volume overload, i.e., too much blood in the left ventricle. During subsequent ventricular systole (when the heart contracts), there is more blood in the ventricle than expected. This means that: (1) the heart must pump harder to move the extra blood; (2) too little blood may move from the heart to the rest of the body; and (3) over time, the left ventricle may begin to stretch and enlarge to accommodate the larger volume of blood, and the left ventricle may become weaker.
Although mild cases of mitral valve regurgitation result in few problems, more severe and chronic cases eventually weaken the heart and can result in heart failure. Mitral valve regurgitation can be an acute or chronic condition. It is sometimes called mitral insufficiency.
III. Prior Treatment Modalities
In the treatment of mitral valve regurgitation, diuretics and/or vasodilators can be used to help reduce the amount of blood flowing back into the left atrium. An intra-aortic balloon counterpulsation device is used if the condition is not stabilized with medications. For chronic or acute mitral valve regurgitation, surgery to repair or replace the mitral valve is often necessary.
To date, invasive, open heart surgical approaches have been used to repair or replace the mitral valve with either a mechanical valve or biological tissue (bioprosthetic) taken from pigs, cows or horses.
The need remains for simple, cost-effective, and less invasive devices, systems, and methods for treating dysfunction of a heart valve, e.g., in the treatment of mitral valve regurgitation.